Lithium silicate glass-ceramic blanks have been proven their value in the field of dental prosthetics due to their strength and biocompatibility. There is the advantage that, if a lithium silicate blank comprises lithium metasilicate as main crystalline phase, an easy machining is possible without undue wear of tools. If then a heat treatment takes place, during which the product is converted into a lithium disilicate glass-ceramic, a high strength is obtained. Also good optical properties and a good chemical stability are given. Respective methods are disclosed in DE 197 50 794 A1, or DE 103 36 913 B4.
It has been shown that the strength is increased and a good translucence can be reached, if at least a stabilizer selected from the group zirconium oxide, hafnium oxide, or mixtures thereof, especially zirconium oxide, is added to the starting raw materials in form of lithium carbonate, quartz, aluminum oxide, etc., which are usual initial components. The percentage by weight of the stabilizer in the initial composition can be as high as 20%. In this respect, reference is made to DE 10 2009 060 274 A1, or WO 2012/175450 A1, WO 2012/175615 A1, WO 2013/053865 A2, or EP 2 662 342 A1.
In practice, however, problems arise in such a way that after the final crystallization, i.e., especially at the stage when lithium disilicate is existent as main crystalline phase in the lithium silicate glass, the stabilizers, especially zirconium oxide, recrystallize although the latter had been fully dissolved in the glass phase of the lithium silicate before.